Correlated spatial variation in allele frequencies and zygotic proportions at the leucine aminopeptidase locus in Modiolus demissus and Mytilus edulis will be studied on both a macro and microgeographic level. Groups of individuals of both species will be transplanted from areas of a particular genetic composition to areas of differing genetic composition. Patterns of mortality of specific genotypes as well as correlated temporal mortality will be determined. Differential mortality at the leucine aminopeptidase locus will be investigated under controlled experimental conditions with individuals held in salt water tanks and subjected to low environmental temperatures. Allozymes at the leucine aminopeptidase loci of both species will be purified by affinity chromatography and functionally characterized with respect to temperature dependent properties of specific activity, maximum velocity, substrate inhibition, temperature stability, and so forth.